Highly integrated semiconductor circuits are increasingly important, particularly in producing battery operated devices such as cell phones, portable computers such as laptops, notebook computers and PDAs, wireless email terminals, MP3 audio and video players, portable wireless web browsers and the like, and these sophisticated integrated circuits increasingly include on-board data storage.
As is known in the art, process variations that occur during manufacture of the semiconductor substrates result in different characteristics for devices on different wafers, or for devices on different portions of the same wafer. Of particular interest to the qualification of parts made using semiconductor substrates are characteristics that relate to temperature sensitivity or temperature dependent properties. So called “process corner analysis” may rate a wafer or dies obtained from a lot by determining from certain process dependent characteristics that are monitored by test equipment, and the results indicating whether the devices are fast, slow, and more or less sensitive to temperature variations, etc.
For example, threshold voltages of transistors on the wafer may be an important, temperature dependent characteristic. Another measure often considered is the current through MOS devices in saturation, typically referred to as IDSAT. This current may also vary with process variations, so in order to determine whether a particular device manufactured on a semiconductor substrate meets the quality criteria established for these parameters, testing is usually performed on the wafer, on individual dies, or on a packaged integrated circuit to observe these characteristics.
Temperature on the semiconductor device is another important measure. When temperature dependent characteristics are measured, the absolute temperature on the semiconductor device is of interest. In the prior art, a thermal sensor using, for example, a proportional to absolute temperature (PTAT) circuit element may be provided on an integrated circuit, or in a scribe line on a semiconductor wafer. It is also known to perform an analog to digital conversion of the resulting current/voltage and output a digital signal that has a known correspondence to the analog temperature value of interest. However, the need for increasing ease and speed in measuring the process dependent characteristics of a completed device, as well as the need for performing these measures in the digital domain to increase efficient use of valuable tester time, thereby improving throughput, remains. A fast and easily accessible readout of important measured process dependent performance characteristics to enable process corner determinations for wafers or individual integrated circuits or dies, without the need for complex test equipment, is needed.
Thus, there is a continuing need for an efficient and effective process monitor circuit and process monitor methods that provide reliable and easy to use outputs for various device characteristics associated with semiconductor process variations without the need for extensive testing or large numbers of wafer or circuit probes.